if you had to choose one activity....

Now being as we're on a cycling forum, I can probably guess the answer but over the years I really have found no better sport than cycling for preparing me for other activities which include skiing, racket sports, walking and to some extent running.

Now there is no substitute for a sport other than the sport itself but cycling gives me a good level of Cardio-Vascular fitness, good endurance and uses a lot of the muscle groups for skiing and walking/running and because you use you arms and upper body when sprinting climbing etc you even get some benefit for racket sports.

I can't think of any other activity I've done which gives me the overall tone of cycling.

Once I would have said cross-country skiing.
But now, cycling; it's not just a means of getting/staying fit and healthy, it's a way of life, a method of travelling, a sporting pursuit/hobby, and, mostly for me, total escapism - if only for a few hours/days/weeks. But I hope to build up to months, in the nearish future.